Polymers of lower α-alkenes or olefins such as ethylene, propylene or 1-butene find applications in the manufacture of a variety of articles including plastic bags or sheets or automobile parts. Of particular interest in polymer production is polypropylene with a high degree of isotacticity i.e. the extent of orientation of the branched groups in the polymer in the same direction, which shows high crystallinity. The polymerisation involves reacting the lower α-alkene such as propylene with a catalyst under polymerisation conditions. The early polymerisation catalysts were of relatively low activity and the polymers formed contained significant amounts of the catalyst residues which had to be removed by deashing steps. The more recent alkene polymerisation catalysts are of two types viz the single site catalysts and the heterogeneous solid catalysts. The single site catalysts comprise metallocenes or co-ordination complexes of transition metals and a cocatalyst such as methyl alumino oxane. These catalysts show activity when used in solution.
Heterogeneous solid catalysts are the most commonly used catalysts, specially in the bulk production of polyethylene or polypropylene, due to their high activity. A heterogeneous solid catalyst comprises a procatalyst, a cocatalyst and a selectivity control agent. The cocatalyst may be an organoaluminium compound such as alkyl aluminium. The selectivity control agent may be an ester such as p-substituted benzoate or phthalate ester or an ether such as alkyl, alkoxy or aryl alkoxy silane. The physicochemical properties of the procatalyst plays a pivotal role in the overall performance of the catalyst, particularly in producing polymers of high isotacticity index. The procatalysts are synthesised by halogenation of an organomagnesium compound such as magnesium ethoxide with a halogenating agent such as titanium tetrahalide in a hydrocarbon or halohydrocarbon solvent such as toluene or chlorobenzene to form magnesium chloride. The magnesium chloride so obtained is reacted with titanium alkoxide or excess titanium tetrahalide, usually titanium tetrachloride in the presence of a hydrocarbon/halohydrocarbon solvent. To this an internal electron donor such as an ester, for example ethyl benzoate, is added simultaneously or sequentially to result in a procatalyst. (U.S. Pat. Nos. 4,535,068, 4,414,132, 4,400,302, 4,477,639, 4,497,905, 4,535,068, 4,657,995, 4,710,482, 4,728,705, 4,771,024, 4,804,648, 4,870,039, 4,914,069, 4,870,040, 5,066,737, 5,077,357, 5,106,806, 5,082,907, 5,122,494, 5,124,298, 5,141,910, 5,151,399 and 5,229,342). In most of the above processes employing organomagnesium compounds, halogenation is usually carried out with titanium tetrahalide itself, because of which these routes necessitate use of higher amounts of titanium reagents in the preparation of the procatalyst, thereby rendering these processes expensive. Besides, organomagnesium compounds such as magnesium alkoxides in the presence of titanium compounds may result in by-products. Since an ester as such is separately added to the reaction mixture, there are chances of the free ester being present, which will react with the titanium tetrahalide [“Reactions of titanium tetrachloride with diesters”, J Organomet. Chem. (1993), 443(1), 85-91 by Sobota Piotr etal; “Synthesis and crystal structure of the titanium tetrachloride-ethyl propionate adduct”, Z Kristallogr. (1991), 194 (3-4), 267-72]. This reaction is exothermic and generates by-products in the preparation of the procatalyst which may affect the performance/activity of the catalyst.
Procatalysts may also be prepared by a method comprising milling together anhydrous magnesium chloride, a titanium tetrahalide such as titanium tetrachloride and an internal electron donor such as an ester in a hydrocarbon or halohydrocarbon solution at ambient temperature to produce a precursor. The precursor is then reacted with titanium tetrahalide to produce the procatalyst (U.S. Pat. Nos. 4,329,253 and 4,393,182). In yet another method, anhydrous magnesium chloride is milled with an internal electron donor such as an ester to produce a precursor which is milled with titanium tetrahalide (U.S. Pat. No. 4,419,501). Anhydrous magnesium chloride used in the above methods is expensive and not readily available. Procatalysts formed by these physical methods when used in polymerisation reactions, may show poor selectivity and result in polymers with low isotacticity index. In this process also by-products formation due to reaction of titanium tetrahalide with free ester cannot be negated. Due to use of anhydrous magnesium chloride, additional steps such as spray drying or melting may be required which render the process time consuming. Also the first method demands use of large amounts of titanium reagent, firstly for halogenation of magnesium chloride and then for being supported on the precursor. The use of excess titanium reagent makes the process expensive.
U.S. Pat. No. 4,948,770 describes a process for the preparation of a heterogeneous solid procatalyst comprising titanium compound such as titanium tetrahalide supported on a precursor obtained by reaction of magnesium halide alcoholate formed of anhydrous magnesium chloride and an aliphatic alcohol such as ethanol or isobutanol with anhydrous silica, and an internal electron donor such as an ester.
U.S. Pat. No. 4,535,068 describes a process for the preparation of a heterogeneous solid procatalyst comprising titanium tetrahalide supported on a precursor obtained by halogenation of an organomagnesium compound such as magnesium diethoxide with titanium tetrahalide and an internal electron donor such as an ester in a halohydrocarbon solvent and reacting the resulting halogenated magnesium compound with an acid halide such as benzoyl chloride. Although not specifically stated in the above patent, addition of acid halide is expected to be for the removal of by-products formed during the course of the reaction. In the above process also, since an ester is as such separately added to the reaction mixture containing titanium tetrahalide, there are chances of by-product formation by reaction of ester with titanium tetrahalide. Organomagnesium compounds in the presence of titanium tetrahalide may also result in by-products and affect the performance of the catalyst. This route also employs a two-stage use of titanium reagent and is expensive.